1. Field of the Invention
This invention relates to a signal processing apparatus for performing a scramble process in transmission or storage of digital coded signals and a descramble process in reproduction of the coded signals in such a manner that permits only authorized users who are given with a decoding procedure to reproduce the coded signals.
2. Description of the Prior Art
Scramble systems have been employed for pay TV broadcasts and the like. A scramble system for digital signals was introduced in a technical report by Nezu and other researchers titled as "A scramble system-M-for communication satellite television" ITEJ Technical Report Vol. 15, No. 14, pp. 1 to 6, in which descriptions are made on the scramble of sound signals. With the conventional scramble system, a signal processing apparatus comprises an exclusive OR circuit for adding a random number to an input signal, a random number generator for generating the random number in response to a scramble key, and a controller disposed between the exclusive OR circuit and the random number generator to control the random number value to be added to the input signal. The input signal is bit-inverted by logical exclusive addition to a pseudo random number from a pseudo random number generator in the exclusive OR circuit thereby to become a random-numbered, or scrambled, signal. In this case, the controller controls the bit inversion rate within the input signal by changing whether or not the random number is added to the input signal at an appropriate rate. The decoding thereof may be made by logical exclusive addition of a random number from the random number generator to the scrambled signal again. The pseudo random number generated by the random number generator is determined in response to the scramble key, and only a decoder having the scramble key can achieve decoding. With the signal processing apparatus for performing scramble and descramble as conventionally, under the condition that the synchronization on both the scramble side and descramble side can be made comparatively easy, that is, under the condition that the scrambled position within the data on the descramble side can be detected easily independently of the bit pattern of data, even if data to be scrambled are made of random numbers, the synchronization can be easily taken on the descramble side, thus being capable of reproducing the data correctly.
With the conventional structure, however, in such a communication system that data are received on the reception side from the midway of a consecutive data sequence and the synchronization of reproduction is effected by using a reserved word having a specifically reserved bit pattern, there would occur a case that a bit pattern coincided with the reserved word be generated within the scrambled data. In the case of descrambling such scrambled signal, the bit pattern coincided with the reserved word within the scrambled signal causes an incorrect synchronization, resulting in malfunction of the circuit on the reproduction side and arising such a problem that reproduction cannot be achieved correctly.